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Abstract: Nitric acid (HNO3) and ozone (O3), secondary products of photochemical reactions of nitrogen oxides (NO x ) and volatile organic compounds, are important pollutants in arid regions with large outputs from petrol combustion. In the Los Angeles (LA) air basin, nitrogen dry deposition rates in forests downwind of the urban areas can reach 35–40kgha−1 year−1, roughly equivalent to the amount of N used to fertilize agricultural fields. The marked decline in the lichen population of the LA air basin has previously been attributed to local O3 concentration gradients, which overlaid the patterns of species extirpation. Recent research in the air basin has shown that nitrate (NO3 −) deposition gradients run parallel to the O3 concentration gradient, and that deposition of NO3 − and HNO3 can have significant effects on forest health. Our research examines the effects of HNO3 dry deposition on the lichen Ramalina menziesii Tayl. in an effort to understand the loss of lichen species in southern California, and increase the usefulness of lichens as biomonitors of nitrogen pollutants. We transplanted healthy R. menziesii thalli from a “pristine” location into fumigation chambers and exposed them to HNO3 under humid and dry conditions, and moderate and high HNO3 fumigations. R. menziesii thalli treated with HNO3 in month-long fumigations experienced a significant decline in chlorophyll content and carbon exchange capacity compared to thalli in control chambers. Leachate conductivity, NO3 − and K+ concentrations increased with HNO3 fumigation levels and time. We conclude that R. menziesii has an unequivocally negative response to HNO3 gas concentrations common to ambient summer conditions in the LA air basin. [Copyright &y& Elsevier]